1. Field of the Invention
The present invention relates to processes for detecting moving underwater objects by means of an active sonar, comprising a directional antenna, by using the Doppler effect attached to the relative movement of the object and of the sonar and by forming directional channels on the basis of the signals from the transducers of the antenna.
2. Discussion of the Background
In order to detect a moving object, called, target a with a sonar, it is known practice to use the Doppler effect produced by the movement of the target. In such processes of the prior art, a pulse of narrow bandwidth compared with the Doppler shift from the target is transmitted, then on reception, the received signals are simultaneously correlated with several frequency-shifted copies of the transmitted pulse. Each correlation copy corresponds to a different possible Doppler shift. The best correlation is obtained with a copy having a frequency shift approximately equal to that caused by the movement of the target. Thus, the correlation by many copies and the use of the signals received make it possible to locate a target by distance and by azimuth and to calculate its radial velocity.
This process amounts to transmitting a bandwidth code which is narrower than the Doppler shift from the targets that one is seeking to detect. To do this, the transmission consists of a pulse of pure frequency f.sub.0 and duration T, amplitude-weighted in order to reduce the level of the secondary lobes of the spectrum transmitted so as to obtain good spectral rejection. The spectral width of such a pulse is then about 4/T for a cos.sup.2 weighting.
It is known that the marine environment is reverberant, especially because of the many local heterogeneities (air bubbles, particles, plankton, etc.) forming scatterers. In addition, at shallow depths, the reverberation coming from the bottom and from the surface is significant. It follows that, when the spectrum of the reverberated signal and the spectrum of the copy are superimposed in the angular sector corresponding to the main lobe of the antenna, the detection performance is very poor.
FIG. 1 shows the value of the frequency f of the signal received as a function of the cosine of the angle .theta. between the velocity vector of the sonar carrier and the direction of a point in space in the bearing plane.
As the carrier moves with a uniform velocity V and as the transmitted frequency is f.sub.0, it is known that the received frequency is given by (1+2.vertline.V.vertline./c cos .theta.)f.sub.0 where c is the speed of the acoustic waves in the water. The spread of the spectrum of the signal reverberated by the entire volume subjected to the sound is therefore represented by a sloping straight line 101 of width 4/T. As to the copy, this is independent of .theta. and is shown by a vertical straight line 102 of width 4/T.
The region denoted A corresponds to the reverberation case indicated above. In this region, the reverberated signal is received in the main receiving lobe 103. It is not removed either by the directivity, nor by the Doppler filtering.
The regions denoted B correspond to the case in which the spectrum of the reverberated signal and that of the copy are superimposed opposite the secondary lobes 104 of the receiving channel. There are therefore two contributions to the reverberated intensity detected. The first is that of the scatterers in the main lobe of the receiving channel, but at frequencies different from the target. These scatterers are rejected by spectral analysis. As the latter can reach 40 to 50 dB in sonar, this contribution can be ignored. A second contribution corresponds to the scatterers at the same frequency as the target, but attenuated by the secondary lobes of the directivity pattern. The situation in the figure shows the intersection 105 of the straight lines 101 and 102 with the secondary lobe 106.
The "reverberation/signal" ratio is given by the formula: ##EQU1##
where R is the distance from the target and NS is the level in dB of the secondary lobes of the directivity pattern. PA1 where a is an integer between 1 and 2. PA1 where .DELTA. corresponds to the smallest interval separating the arithmetic series of the p/q ratio from the series of integers.
The angular interval .DELTA..theta. corresponding to the spectral overlap between the copy and the reverberated signal is such that .DELTA. cos .theta.=.lambda./VT and the ratio (1) does not depend on the duration T of the transmitted pulse: the fact of increasing this duration would not allow the performance to be increased.
The regions denoted C correspond to the case in which there is no scatterer at the frequency of the receiving channel. In this case, the performance is generally very good, but it only corresponds to a limited number of potential targets.
Patent application Ser. No. 92/01499, filed on Feb. 11, 1992 by the Thomson-CSF company and published on Aug. 13, 1993 under U.S. Pat. No. 2,687,226 describes a process for detecting moving targets in which a series of pulses at pure frequencies is transmitted. Its drawbacks stem from the fact that the performance in regions B remain poor and that the transmitted frequencies depend on the speed of the target.